Cement clinker

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Cement clinker

Cement clinker

Cement clinker ( Portland cement clinker , short: clinker) is the burnt component of the cement that is responsible for hardening when water is added. The name clinker comes from the similarity to high-fired ( sintered ) bricks ( clinker ). However, cement clinker should not be confused with the brick product .

Clay minerals and lime are mainly used as starting products, mostly in the form of clay and calcareous rocks such as limestone or marl . The raw materials are ground and then heated at around 1,450 ° C until they partially fuse together ( sintering ). The now spherical material is cooled and ground again to obtain cement . Quartz sand and materials containing iron oxide can be added before firing to improve sintering. In order to obtain cement types with certain properties, blast furnace slag , fly ash and gypsum can also be added, for example .

The four phases of clinker essentially arise during sintering:

• Tricalcium Silicate (Alit) C3S
• Dicalcium silicate (belite) C2S
• Tricalcium aluminate C3A
• Tetracalcium aluminate ferrite C4AF

The clinker mainly consists of the following phases :

Clinker phase	chemical formula	Short formula
Tricalcium silicate	${\displaystyle \mathrm {3CaO\cdot SiO_{2}} }$	${\displaystyle \mathrm {C_{3}S} }$ Alite
Dicalcium silicate	${\displaystyle \mathrm {2CaO\cdot SiO_{2}} }$	${\displaystyle \mathrm {C_{2}S} }$ Belite
Tetracalcium aluminate ferrite	${\displaystyle \mathrm {4CaO\cdot (Al_{2}O_{3},Fe_{2}O_{3})} }$	${\displaystyle \mathrm {C_{4}(A,F)} }$ Brownmillerite
Tricalcium aluminate	${\displaystyle \mathrm {3CaO\cdot Al_{2}O_{3}} }$	${\displaystyle \mathrm {C_{3}A} }$ Calcium aluminate
free CaO	${\displaystyle \mathrm {CaO} }$	- Free lime

The most important phase for hardening is the tricalcium silicate, which has the highest initial hardness.

From a chemical point of view, Portland cement clinker consists of the following main components:

component	Average share [% by mass]
${\displaystyle \mathrm {CaO} }$	66.5
${\displaystyle \mathrm {SiO_{2}} }$	21.5
${\displaystyle \mathrm {Al_{2}O_{3}+TiO_{2}} }$	05.5
${\displaystyle \mathrm {Fe_{2}O_{3}} }$	02.5

The main component alite is formed in the presence of a melting phase from around 1250 ° C. In order to achieve a sufficiently complete reaction (residual free lime content below 1.5%), temperatures of around 1450 ° C. are necessary for a period of 5 minutes; there should prevail a predominantly oxidizing atmosphere in the combustion unit and the precursors belite and free lime are homogeneously.